Housing
What size is your tank?
2.5 gallons
What temperature is your tank?
79 F
Does your tank have a filter?
Yes
Does your tank have an air stone or other type of aeration?
no
Is your tank heated?
yes
What tank mates does your betta fish live with?
He lives alone

Food
What type of food do you feed your betta fish?
Dried blood worms and betta flakes
How often do you feed your betta fish?
Once a day

Maintenance
How often do you perform a water change?
33% every 4 or so days
What percentage of the water do you change when you perform a water change?
About a third
What type of additives do you add to the water when you perform a water change?
Betta water treatment from nutrafin

Water Parameters:
Have you tested your water? If so, what are the following parameters?
I don't have a testing kit or a car or a pet store within biking distance :( I do, however, have a live plant in the tank, so that should clear up some of this?

Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed?
His pectoral fins used to be round and full. Now they're short, clipped, with a jagged straight edge. They went from big circles to small triangles. Also, there's a little light patch the color of his body (he's got green scales and red everything else) on the end of one of his ventrals, but that's been there a while.
How has your betta fish's behavior changed?
Not at all-- he's still eating and playing just fine.
When did you start noticing the symptoms?
I just noticed it today.
Have you started treating your fish? If so, how?
No.
Does your fish have any history of being ill?
Not as far as I'm aware, but he isn't mine.

What do you guys think? It looks like he tore them apart somehow, but why would he do that? The rest of his fins look fine...

I don't think Betta Fish do that... They need the Pectorals for balance and navigation. Can you post a picture? And keep the water extra clean to heal. Oh, and could you change your water change schedule to 1 100% and 1 50% per week? Ammonia levels can get high in that size of a tank. Just be careful with your live plant.

Do you think it would be ok to do 2 75% instead? He hates to be caught, so I use a gravel vacuum so he can stay in his tank while I clean it. I worry catching him once a week would stress him out a lot. I'll grab a picture asap.